Orbital multispectral mapping of Mercury with the MESSENGER Mercury Dual Imaging System: Evidence for the origins of plains units and low-reflectance material

摘要: Abstract A principal data product from MESSENGER’s primary orbital mission at Mercury is a global multispectral map in eight visible to near-infrared colors, an average pixel scale of 1 km, acquired by the Dual Imaging System. The constituent images have been calibrated, photometrically corrected standard geometry, and projected. Global analysis reveals no spectral units not seen during flybys supports previous conclusions that most variation related changes slope reflectance between end-member high-reflectance red plains (HRP) low-reflectance material (LRM). Comparison color properties mapped on basis morphology shows two largest unambiguously volcanic smooth deposits (the interior Caloris northern plains) are close HRP end members distinct those other intercrater plains. In contrast, smaller nearly indistinguishable their range properties, consistent with interpretation older equivalents LRM having same exposed crater basin ejecta all ages, suggesting impact excavation depth intrinsically dark or darkens very rapidly, rather than gradual darkening purely space weathering. search definitive absorptions attributable Fe 2+ -containing silicates sulfides over regions 20 km more horizontal extent, results Atmospheric Surface Composition Spectrometer. only absorption-like feature identified broad upward curvature spectrum centered near 600 nm wavelength. strongest freshly weak absent exposures LRM. We modeled spectra as intimate mixtures candidate phases similar 600-nm feature, under assumption grain size 1 μm larger. Sulfides measured date laboratory coarse-grained iron both too bright produce HRP. Ilmenite sufficiently but would require Ti abundances high be MESSENGER X-Ray Spectrometer measurements. Three could responsible for low our analyses. Graphite, amounts upper limits Gamma-Ray Spectrometer, may geochemical models Mercury’s differentiation calling graphite-enriched flotation crust early magma ocean mixing before late heavy bombardment (LHB) into underlying preexisting sulfide altered mix nanophase microphase grains shock impacts, lowering reflectance. Alternatively, iron-bearing carbon late-accreting carbonaceous veneer stirred lower mantle. Decoupling variations major elements probably content crustal silicates, such controlled instead one minor opaque extent